The invention relates to an electric lamp comprising a translucent lamp vessel,
an electric element in the lamp vessel, current supply conductors extending to the electric element,
a lamp cap connected to the lamp vessel and having a sheath portion and a base portion carrying an electric contact member, the electric contact member having a surface through which a current supply conductor is passed to the exterior, which current supply conductor is welded to said surface.
Such an electric lamp is known, for example, from GB 444 958 (1936-3-23).
The lamp according to GB 444 958 has a lamp cap with a contact member at its base portion, which has a sunken portion, through which a current supply conductor is passed to the exterior. The current supply conductor is fixed in the sunken portion with an electric resistance weld. With this lamp, a contact element of a lamp holder, in which the lamp is arranged, touches the contact member of the lamp cap around the sunken portion thereof. This results in that the vulnerable welding connection is not subjected to mechanical load when the lamp is arranged in the lamp holder.
The known lamp has a number of great disadvantages. The lamp must necessarily be provided with an unconventional lamp cap because of the unconventional contact member at the base portion thereof. The contact member at the base portion is very large. In fact, it must be possible for the member to make contact with a mass electrode and with a welding electrode. Moreover, it must comprise a sunken portion. However, conventional lamp caps are in use, in which the contact member has a diameter of only 2.5 mm, for example the E5 lamp cap. Moreover, conventional lamp caps are in use, in which the base portion has two contact members, for example the B22d lamp cap. In this case, there is no room for two contact members of large dimensions. It is further difficult to make a good resistance weld between the current supply conductor and the contact member because these elements are in linear contact with each other during welding. The welding current is consequently distributed over a line, as a result of which it is possible that nowhere along the wire such a high current density occurs that a good weld is obtained.
U.S. Pat. No. 2,892,923 (1959-6-30) discloses a lamp in which a current supply conductor is welded to a contact pin at the base portion, through which said current supply conductor is passed. When establishing the weld connection, a first electrode is in electrical contact with the shaft of the contact pin and a second electrode at a certain distance from said pin is in contact with the free end of the current supply conductor. Between the area at which the current supply conductor is passed out of the pin and a third electrode, a discharge arc is drawn, which welds the current supply conductor to the pin. The welding connection is established by means of the locally molten material proper of the wire and the pin. Because of the contactings with electrodes which have to take place at a certain distance from the welding point, this manner of securing is not suitable for the contact member at the base portion of, for example, conventional Edison and Swan lamp caps.
U.S. Pat. No. 3,897,124 (1975-7-29) also discloses a lamp having a current supply conductor welded to a contact member at the base portion. However, the lamp cap is of an unconventional type. Its contact member at the base portion has a continuous elevated edge at its periphery. The contact member has centrally around an opening an outwardly protuberant edge. When establishing the weld connection between the contact member and the current supply conductor passed through this opening in this member to the exterior, a hollow first electrode is arranged over the contact member so as to be in contact with the elevated edge. Between a second electrode inside the first electrode and the contact member, a discharge arc is drawn. The latter causes the outwardly protuberant edge to melt and to contact with the current supply conductor, which also melts. Due to the flow of the melts, a welding connection is formed, which consists of the material proper of the contact member and the current supply conductor.
The welding connection is sunken with respect to the elevated edge, which must make contact with a contact element of a lamp fitting. The elevated edge thus protects the welding connection from contact with a contact member of a lamp holder. The welding connection has a rough surface and, if in the absence of the elevated edge it should contact a contact element in a lamp holder, it would consequently have an undefined small contact surface with said element. As a variation of this manner of connecting, the said patent specification mentions the same configuration of contact member and welding electrodes, in which the second electrode is a consumable electrode. A welding connection is then established by mixing the material proper of the contact member, the material proper of the current supply conductor and the material of the consumable electrode. This variation with a consumable electrode is not recommended by the patent specification.
This known lamp has the disadvantage that its lamp cap has an unconventional shape, is intended to make contact only at the edge of the contact member with a lamp holder, requires a comparatively large contact member and especially must be touched when establishing the welding connection in order to make a good electrical contact with the first electrode. Another disadvantage is that the energy of the discharge arc can damage the base portion of the lamp cap. For example, the base portion may crack or the material thereof may be decomposed. A risk is that the current supply conductor melts through within the lamp cap under the action of the heat of the discharge arc and loses its contact with the welding connection. This risk is great if the current supply conductor takes the form of a fuse wire, which in the finished lamp must melt upon heat development due to an excessively high current through the lamp.
EP 0 057 006 A.sub.2 (1982-8-4) discloses an H4 car lamp, in which current supply conductors are secured in the same manner to contact members at the base portion of the lamp cap by drawing a discharge arc between an electrode and an outwardly protuberant edge of an opening in a contact member, through which a current supply conductor is passed to the exterior. In the lamp cap of an H4 lamp, the contact members consist of rectangularly bent strips, which extend to a certain distance from the base portion of the lamp cap. In these contact members, there is plenty of room to make contact with a mass electrode in order to draw with a second electrode an arc to establish a welding connection with a melt of the material proper of the contact member and a melt of the material proper of the current supply conductor. When establishing this connection, however, a comparatively high thermal load of the material of the base portion of the lamp cap occurs.
Several Patent Specifications, inter alia U.S. Pat. No. 2,708,702 (1955-5-17), U.S. Pat. No. 2,809,279 (1957-10-8) and U.S. Pat. No. 4,458,136 (1984-7-3), describe the step of connecting a current supply conductor to the sheath portion of a lamp cap by drawing a discharge arc between the free end of the current supply conductor and an electrode, this end of the current supply conductor then melting and the melt of said conductor itself forming a connection between the conductor and the lamp cap. The sheath of the lamp cap offers plenty of room for a mass electrode. When pressing the lamp cap against the lamp vessel, the sheath portion of the lamp cap contacts the current supply conductor, which is passed to the exterior between the lamp cap and the lamp vessel. The cold lamp cap causes the melt to solidify as soon as it touches the lamp cap.
As appears from the citations, attempts have been made already for many decennia to avoid securing current supply conductors to contact members at the base portion of a lamp cap by soldering. A disadvantage of securing to a contact member at the base portion by soldering is the risk of cold flow of the solder in a lamp fitting. As a result, high contact resistances and excessive heat development may occur in the lamp fitting. Very great disadvantages are further the long process duration of establishing a soldering connection. Not only the time required for heating, but also the time required for cooling the solder to solidification is long. In this cooling period, the lamp must be kept stationary in order to avoid that the solder is thrown away. Another very great disadvantage is the necessity of using a flux to cause the solder to flow and thus to adhere. The residues of the flux may give rise to the formation of a leakage path for electric current between contact members or between a contact member and the sheath portion of a lamp cap. As a result, shortcircuit may occur.